When children turn 18 most (depending on the state) are legally adults and responsible for their actions. It may terminate your child’s governmental benefits. You need to know what to adjust well in advance.

Consider the long-term financial situation early, well before age 18. It may include a special needs trust, an update of your will, and strategy to retain public benefits. Consider an ABLE account and a Representative Payee for Social Security. They are a way to protect them financially. 

You won’t be able to make decisions for them. If they can give you power of attorney after they are 18, you can act for and help them. If they can't, you can go to court, but that's complicated and expensive.

Your schedule and any respite care will change. For a smooth transition you need to understand the options before deciding living arrangements after high school. 

They may qualify for different health insurance depending on the services they already receive. This could be through a state tailored Medicaid Waiver program, or private insurance.  

Supplemental Security Income (SSI) provides cash for low-income, disabled or elderly, and impoverished people. When your child turns 18, your income isn’t counted, so if they did not qualify before, they may now. (It requires a medical problem that stops them from working.)

If your child is in a public school system, they’ll have an Individualized Education Program (IEP). From 9th grade on you and your child decide if their path is to a high school certificate, college, or a trade program. If the choice is high school, they may access a transitional program to focus on basic life skills until they are 21 or 22. If they want a job, your state may have an Office of Vocational Rehabilitation to help.